1. Field of the Invention
This invention relates to ethanol fermentation and more particularly to processes for the continuous fermentation of ethanol in a series of fermentation vessels provided with means for recycling fermenting organisms.
2. Description of the Prior Art
With the ever-increasing depletion of economically recoverable petroleum reserves, the production of ethanol from vegetative sources as a partial or complete replacement for conventional fossil-based liquid fuels becomes more attractive. In some areas, the economic and technical feasibility of using a 90% unleaded gasoline-10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to a recent study, gasohol powered automobiles have averaged a 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficient fuel, biomass-derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain petroleumbased chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantity and versatility.
The various operations in processes for obtaining ethanol from such recurring sources as cellulose, cane sugar, amylaceous grains and tubers, e.g., the separation of starch granules from non-carbohydrate plant matter and other extraneous substances, the chemical and/or enzymatic hydrolysis of starch to fermentable sugar (liquefaction and saccharification), the fermentation of sugar to a dilute solution of ethanol ("beer") and the recovery of anhydrous ethanol by distillation, have been modified in numerous ways to achieve improvements in product yeild, production rates and so forth. For ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient in the use of energy as possible so as to maximize the energy return for the amount of ethanol produced and enhance the standing of the ethanol as an economically viable replacement for petroleum based raw materials. To date, however, relatively little concern has been given to the energy requirements for manufacturing ethanol from biomass and consequently, little effort has been made to minimize the thermal expenditure for carrying out any of the discrete operations involved in the manufacture of ethanol from vegetative sources.
The substitution of alcohol for at least a portion of petroleum based fuels is particularly critical for developing economies where proven domestic petroleum reserves are limited, such as in India and Brazil and these nations have therefore increasingly emphasized the production of alcohol from vegetative sources.
Processes for the continuous fermentation of sugars to provide alcohol and ethanol fermentation processes featuring yeast recycle are known (viz., U.S. Pat. Nos. 1,201,062; 2,054,736; 2,063,223; 2,122,939; 2,146,326; 2,169,244; 2,230,318; 2,272,982; 2,285,130; 2,155,134; 2,371,208; 2,657,174; 2,676,137; 2,967,107; 3,015,612; 3,078,166; 3,093,548; 3,177,005; 3,201,328; 3,207,605; 3,207,606; 3,219,319; 3,234,026; 3,413,124; 3,528,889; 3,575,813; 3,591,454; 3,658,647; 3,676,640; 3,705,841; 3,737,323; 3,940,492; and 3,984,286; "Process Design and Economic Studies of Alternative Fermentation Methods for the Production of Ethanol", Cysewski, et al. Biotechnology and Bioengineering, Vol. xx, Pp. 1421-1444 (1978)). In the continuous fermentation process described in U.S. Pat. No. 3,234,026, referred to above, a stream of sterile sugar liquor and a quantity of yeast cells are introduced into the first of a series of fermentation vessels wherein initial fermentation is carried out, generally under conditions favoring rapid cell growth. Partial fermentate is continuously withdrawn from the first fermentation vessel and is intruduced into a second fermentation vessel wherein fermentation is carried out under conditions favoring rapid conversion of sugar to ethanol. Effluent from the second fermentation is separated into a yeast stream and a subsantially yeast-free ethanol ("beer") stream with the yeast stream being recycled to the first fermentation vessel. In this and similar fermentation processes, practical difficulties are encountered in maintaining optimum levels of yeast in both fermentation vessels since the quantities of yeast available for recycle are directly tied to conditions of fermentation which happen to exist in each vessel at a particular point in time. Accordingly, there has heretofore existed a need for a process of continuously fermenting sugar to ethanol in a series of fermentation vessels in which optimum levels of yeast in each vessel are readily and conveniently maintained without regard to the specific fermentation conditions in the vessel at a given moment.